Radio over fibre, RoF, in combination with Sub-Carrier Multiplexing, SCM, technology is a promising solution to feed several radio antennas on a common optical fibre due to the large available bandwidth. A number of radio signals are combined together and used to modulate an optical carrier to be transmitted over optical fibre. SCM can be used to combine many radio signals on a single wavelength, and wavelength division multiplexing, WDM, enables many wavelength channels to be multiplexed into a single optical fibre. The combination of WDM and SCM provides an efficient, potentially high-performing solution for mobile fronthaul, in particular when there is a high-density of radio antenna heads/cells.
As reported in C. Xu et al, “Performance improvement in RoF linked based on optical carrier suppression using a phase-shifted FBG”, Proceedings of the Asia Communications and Photonics Conference 2013, ATh3G.3, suppressed carrier SCM transmission provides an increased optical power budget and better optical link performance. In addition, the application of photonic integration can enable cost-effective implementation of arrays of transceivers with active elements (such as semi-conductor optical amplifiers and WDM optical sources).
In RoF, the overall system performance is limited by optical subsystem blocks, mainly due to the nonlinear transfer function of the electro-optical modulator in each transceiver. It is essential to provide high spurious free dynamic range, SFDR. Actual systems, except for those which are too expensive to be deployed for this kind of application, have poor performance in term of SFDR. Conventional SCM techniques leave most of the optical power in the optical carrier baseband, limiting SFDR and optical power budget. To improve the power per subcarrier, and thus provide higher SFDR, one can reduce the optical carrier and afterwards optically amplify the signal, as reported by C. Xu et al, supra. The effectiveness of the carrier suppression technique requires a precise power calibration of the ratio between carrier and subcarriers and then a proper adjustment of the optical amplifier gain.
Current mobile fronthaul networks are not reconfigurable in the optical domain. This is a major limitation for network operators since they are not able to allocate dynamically network resources, such as wavelengths or subcarriers, according to their needs. Moreover the installation process requires skilled technicians and huge inventory costs typical of WDM systems. Analog systems have many advantages in terms of latency control and easier centralization of processing resources, but they are typically not robust to noise and distortions; for example, intermodulation products due to non linearities. SCM based on partial carrier suppression offers an improvement in terms of robustness to distortions and noise, but it is difficult to choose the optimal power ratio between optical carrier and SCM subcarriers because it changes from system to system and over time.